Joint diseases are a major cause of disability and early retirement in the industrialized countries and are thus of great socioeconomic significance. Of the joint diseases, osteoarthritis (OA) has by far the greatest prevalence, and it has been calculated that, in the United States, OA is responsible for the consumption of up to thirty times more sick-leave days or hospital days than rheumatoid arthritis (Kramer, J. S., E. H. Yelin, W. V. Epstein [1983] Arthritis Rheum. 26:901-907). OA is a slowly progressive disease of multifactorial etiology. The rate of disease progress will vary greatly between different patients, depending on the underlying pathogenic factors. Consequently, progress from the very early stages to the overt, clinical stages may take anything from years to decades.
The diagnostic criteria for OA are currently based on the clinical presentation and obligatory radiographic signs (Altman, R. D., J. F. Fries, D. A. Bloch et al. [1987] Arthritis Rheum. 30:1214-1225). Since the radiological diagnosis is usually based on a decreased "joint space," it depends on the actual destruction of joint cartilage and will therefore be made only late in the disease. We lack routine methods to diagnose "preOA" or "preradiological" stages of OA, a reflection of our lack of techniques to monitor the joint cartilage in vivo. We are thus unable to determine the ongoing disease activity or the prognosis for the patient threatened by joint cartilage destruction. Moreover, we are unable to monitor with any precision or specificity the effect of pharmacological or surgical intervention aimed at retarding or reversing cartilage destruction in OA or other joint diseases. Any new and improved techniques to diagnose and follow OA need to monitor the present in vivo state of health of the cartilage, not only provide a historical record of past destructive disease.
The details of the mechanisms involved in the disease process of OA are not known. Presumably, the pathogenesis is multifactorial, with genetics, joint malalignment, joint overload or trauma, obesity, and aging as some of the known or suspected contributing factors. Even less well known is how these general factors are translated into disease mechanisms on the tissue and cell level. It may also be that the initiation and progression of OA are controlled by different factors. Since, however, changes in the properties of joint cartilage and loss of matrix components are an integral pan of the disease process, it can be argued that degradation of cartilage matrix is a key event at some time in the development of OA. During this process, matrix molecules, or fragments thereof, are released to the joint fluid and eventually to other body fluids. These molecules and fragments could be used as markers of cartilage turnover in OA and other joint diseases (Lohmander, S. [1988] Clin. Rheumatol. 2:37-62; Lohmander, L. S. [1990] "Cartilage markers in joint fluid in human osteoarthritis," In: Brandt, K., ed. Cartilage changes in osteoarthritis, Indianapolis: Indiana University School of Medicine Press (ISBN 0-914168-90-8), pp. 98-104; Lohmander, L. S. [1990] "Osteoarthritis: Man, Models, and Molecular Markers," In: Maroudas, A., K. Kuettner, eds. Methods in Cartilage Research, London: Academic Press, pp. 337-340).
The aggregating proteoglycan of articular cartilage, or aggrecan, is composed of a protein core to which is attached chondroitin sulfate, keratan sulfate, and both N-linked and O-linked oligosaccharides. The protein core has an extended central segment, to which the glycosaminoglycan chains are attached. At the NH.sub.2 terminus, two globular domains, known as G1 and G2, are separated by a short segment known as the interglobular domain (IGD). At the COOH terminus, a single globular domain, G3, is found. The G1 domain is involved in the binding of aggrecan to hyaluronan and link protein, an interaction that probably serves to immobilize the proteoglycan in the tissue (Doege, K., M. Sasaki, T. Kimura, Y. Yamada [1991] J. Biol. Chem. 266:894-902).
The catabolism of aggrecan in cartilage explants has been found to involve limited proteolysis of the core protein with the release from the tissue of large chondroitin sulfate-bearing species. Analysis of these major catabolic products with antibodies to the G1 domain and to keratan sulfate (Ratcliffe, A., J. Tyler, T&gt;Hardingham [1986] Biochem J. 238:571-580) have indicated that such proteolysis separates the G1 domain from the remainder of the molecule. Interleukin-1.alpha. (IL-1.alpha.) has been shown to induce increased catabolism of aggrecan in cartilage explants, and high levels of IL-1 in human joint effusions may be responsible for the cartilage degeneration seen in inflammatory joint diseases.
The disease mechanisms active in OA are unclear, but changes in the biochemical and biomechanical properties of joint cartilage, changes in chondrocyte matrix synthesis, and finally, a gradual destruction of the matrix are characteristic of the disease process. Cartilage proteoglycan fragments are released to joint fluid after knee injury and in early stages of both posttraumatic and primary OA in the human (Lohmander, L. S., L. Dahlberg, L. Ryd, D. Heinegard [1989] Arthritis Rheum. 32:1434-1442; Lohmander, L. S., L. Dahlberg, L. RYD, D. Heinegard [1990] Trans. Orthop. Res. Soc. 15:212 [abstr.]; Lohmander, L. S., L. Dahlberg [1991] Trans. Orthop. Res. Soc. 16:227 [abstr.]). An important role for matrix metalloproteinases in both normal turnover of connective tissue matrix and in the tissue destruction seen in, for example, OA has been suggested (Docherty, A. J. P., G. Murphy [1990] Ann. Rheum. Dis. 49:469-479; Murphy, G., R. M. Hembry, C. E. Hughes, A. J. Fosang, T. E. Hardingham [1990] Biochem. Soc. Trans. 18:812-815), and an imbalance between tissue metalloproteinases and inhibitors has been demonstrated in animal model OA cartilage, in joint fluids from patients with recent joint injury, and in OA joint fluids. Further, increased expression of mRNAs for collagenase, stromelysin, and metalloproteinase inhibitor has been shown in synovial cells stimulated with interleukin-1 and in synovial cells from rheumatoid or osteoarthritic joints (McCachren, S. S. [1991] Arthritis Rheum. 34:1076-1084; Firestein, G. S., M. M. Paine, B. H. Littman [1991] Arthritis Rheum. 34:1094-1105). However, no definitive evidence has yet been presented which demonstrates that these enzymes are directly involved in cartilage matrix breakdown in OA. A detailed characterization of matrix fragments released from joint cartilage after trauma and in OA of the human could help identify the degradative mechanisms.
Characterization of aggrecan fragments released from bovine explants treated with interleukin-1 in vitro has demonstrated a major cleavage site within the interglobular domain of the proteoglycan core protein which releases large aggrecan fragments from the tissue (Sandy, J. D., P. J. Neame, R. E. Boynton, C. R. Flannery [1991] J. Biol. Chem. 266:8683-8685).